The present invention relates to an active matrix display and method to drive the display.
Active matrix displays are formed of many light emitting units called pixels. Each pixel includes an electronic circuit that controls a light emitting diode. The pixels are arranged in an array of rows and columns to form a display. In operation, each pixel of an array is sequentially programmed with an updating data value that is transformed into a light level.
In a typical 2-TFT pixel, the data value that determines light intensity is provided externally in the form of a voltage. The voltage is transformed by the pixel circuit into a current that is directed to the organic light emitting diode (OLED). The amount of current determines an amount of diode emitted light. As an OLED is programmed, a thin film transistor (TFT) transmits a data value voltage from a program line to a gate of another transistor that regulates current that flows to the OLED from a power supply.
The current that flows through the current regulating transistor depends on the voltage at its gate. Factors such as the transistor material properties have a direct affect on current flow through a transistor. Transistor material property variations (mismatch) can result in a different current for the same programmed voltage level to two different pixels. This in turn results in a difference in light output. Various pixel designs have been proposed with increased number of transistor and control lines to address this problem. However, these designs are complex structures with reduced yield and aperture ratio.
There is a need for an active matrix display that includes a pixel driver with improved output uniformity that utilizes a minimum of transistors, avoids complex pixel circuitry and that can be rapidly programmed.